This invention relates to electrochemical fuel cells and, more particularly, to fuel cell membrane electrode assemblies and the flow field structures adjacent thereto.
Electrochemical fuel cells generate electrical current through the oxidation of a fuel. One type of fuel cell employs a membrane electrode assembly ("MEA") including a membrane having an anode side and cathode side, depending on the direction of the current with respect thereto. The membrane itself serves as an electrolyte. A suitable catalyst for the electrochemical reaction is applied to the membrane, or is incorporated into the polymeric composition from which the membrane is prepared.
Located on both sides of the MEA is a flow field which typically consists of a graphite plate which has been machined to provide a series of channels on its surface, as shown, for example, in U.S. Pat. Nos. 5,300,370 and 5,230,966. The channels transport fuel to the anode side and oxidant to the cathode side, and transport reaction products from the cathode side, and are typically separated from the membrane electrode assembly by a thin layer of a porous carbon material, such as carbon fiber paper. However, this layer of porous material limits the operating efficiency of the fuel cell, particularly at higher operating currents, by creating a mass transport limitation within the fuel cell. This limitation may be observed on a graph of voltage vs. current density for the fuel cell, as a sharp increase in the slope of the graph (in the negative direction) as the current density increases, as well as a lower overall power density. It would be desirable to increase the operating efficiency of such fuel cells.
It is known to prepare a fuel cell having a layer of a carbon/polytetrafluoroethylene mixture deposited on a graphite cloth or paper prepared from a dispersion of carbon and polytetrafluoroethylene in water. However, the operating efficiency of such fuel cells is still less than desirable.